Electric controls with -transistors



June 8, 1965 S. A. ZARLENG ELECTRIC CONTROLS WITH TRANSISTORS Original Filed June 24, 1958 2 Sheets-Sheet 2 INVENTOR.

57w: 5. Z ar/e/yy BY United States Patent 3 188,500 ELECTRIC CONTRGLS WITH TRANSISTORS Steve A. Zarleng, Akron, Qhio, assignor to The Clark Cgptroller Company, Cleveland, Ohio, a corporation of This application is a division of my copending application, Serial No. 744,251, filed June 24, 1958, now Patent No. 3,070,710.

The invention hereof relates to electric control systems; and relates more particularly to control systems or circuit complexes that operate in a manner to supply output current from a current source to a load to be energized, alternately, at a high value, and at zero value.

Such control systems are known and are sometimes arranged to respond in this manner to the operation and restoring of two contactors. When one contactor is operated, it causes the system to deliver high output and to maintain that high output after the contactor has been restored; and when the other contactor is operated, it causes the system to reduce its output to zero value, and to so maintain it when said other contactor is restored.

The present invention relates more particularly to control systems having the aforesaid characteristics of operation.

In my copending patent application, Serial No. 507,643, now Patent No. 2,966,979, I have shown in FIGURE 1 a control system of this type, and the present invention may be considered as an improvement thereover.

In the control system of that application, two contactors and three transistors are utilized. Upon the operation of one contactor, a first one of the transistors is rendered conducting, and is immediately maintained conducting by a second transistor, and current flows through the first transistor and to a third transistor, rendering it conducting, and current is conducted through the third transistor to a load.

The said one contactor may be restored without changing these conditions.

Then the load current may be reduced to zero value by operation of the other one of the two contactors, which renders the first transistor non-conducting and it is maintained non-conducting when the said other contactor is restored.

The present invention is shown anddescribed hereinafter in two forms, each of which is an improvement over that of my said pending application in that, among other differences, it requires only one transistor.

In the first form, upon operating one of the two contactors the transistor is rendered conducting, and current from a supply source flows at full value through the transistor and through the load; and current is derived from the load current to actuate a feedback transformer, the output of which maintains the transitsor conducting. The said one contactor may then be restored, without changing these conditions.

Then the load current may be reduced to zero value by operating the other contactor, which renders the transistor nonconducting.

As an alternative mode of operation of this form, after the one contactor has been operated and the full value of current flows through the load as described, the one contactor may be maintained operated and not restored; and to reduce the load current to zero value, the said other contactor may be operated to render the transistor nonconducting. 7

Then to again cause full load current to fiow, the said other contactor may be restored. The said one contactor being still operated, the transistor will be rendered conducting thereby, and full current to the load will again how as described.

According to this alternative mode of operation, when the transistor has once been rendered conducting by operating the one contactor, and it is then maintained operated, the transistor may be rendered non-conducting and again conducting, alternately, by alternately operating and restoring the said other contactor.

In the second form of the invention a saturable transformer is provided having two opposed primaries energizable respectively by the aforesaid two contactors; and having a secondary connected to the transistor.

By one mode of operation, upon closing one contactor, one primary saturate-s the transformer, and in doing so, a pulse of secondary current renders the transistor conducting and full value of load current flows through it and current derived from the load current maintains it conducting. The one contactor may then be opened.

The load current may then be reduced to zero value, by closing the other contactor to energize the other primary to cause it to saturate the transformer in the opposite direction which causes a pulse of secondary current to render the transistor non-conducting and stop the how of load current through it. The said other contactor may then be opened.

According to an alternative mode of operation of this second form of the invention, one primary is always energized to saturate the transformer in the direction to cause the transistor to be conducting, and the other primary has sufficient magneto-motive force to overpower that of the one primary and reverse the saturation of the transformer and render the transistor non-conducting.

The load current is therefore caused toflow at full value or to be cut off, respectively, by alternately opening and closing a single contactor that effects de-energization and energization of the overpowering primary.

It is among the objects of the invention:

To provide, generally, an improved control system of the type referred to above;

To provide an improved control system having, among others, features set forth in the foregoing brief description of the invention;

To provide a control system comprising a transitsor in the path of load current from a current source through a load and rendered conducting by initiation of current flow in an activating current path in the transistor in a first direction and maintained conducting by current in said activating path in said direction derived from the load current; and rendered non-conducting by initiation of current in said activating path in the opposite direction.

Other objects will occur to those skilled in the art, from a reading of the following full description of the invention.

An embodiment of the invention and a modification thereof are fully described in the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is a diagrammatic view of an electric system embodying the invention in one form;

FIGURE 2 is a diagrammatic view of a modification of the embodiment of FIGURE 1;

FIGURE 3 is a view illustrating a hysteresis curve, by reference to which, operation of the modification of FIG- URE 2 is explained; and

FIGURE 4 is a view of the embodiment of FIGURE 2 in simplified form.

Referring to FIGURE 1 of the drawing there is shown at 1 the one transistor of the premises.

It is of the PNP type and comprises an emitter 2, a base 3, and a collector 4.

There are two circuits controlling, respectively, conductivity and non-conductivity of the transistor ll; energized with unidirectional potential of opposite polarity, from quadrilateral rectifiers and 6, which receive input from alternating supply mains 7 5.

As to one of said circuits, at half wave of current from main '7 has a path by wire d, through rectifier 6, by wire it), through normally open contacts of one contactor ill, by wires 12; and id to emitter 2; and through the activating circuit of the transistor to base 3,'and thence by wires 35 and 16 and a resistor R1, through the r ctifier 6 and by wire 17 to supply line ti.

The next half wave has a similar path from main 8 by wire 17, through rectifier 6, and thence by wire til, contactor' 13., and again by wire '14 through the emitter 2 and base 3 of the transistor and back by wires i545 and resistor R1, through the rectifier 6 to wire 9 and main 7.

Upon closing contactor 11, full wave rectified current in this circuit will go from emitter 2 to base 3 in the tran-.

sistor and will render the transistor conducting, asis well known.

As to the other of said circuits, at half wave of current from supply main '7 has a path by wire ltd, through the rectifier 5, resistor R2, wires 16' and 15, to the base 3 and from emitter 2 by wires 14, 12 and 19; through normally open contacts of another contactor 20, wire 21 through the rectifier 5, and by Wire 21A to line 8.

Similarly the next'half wave will go from main 3 by wire 21A through rectifier 5, by resistor R2, wires 16-15 to base 3 of the transistor and from its emitter 2, and by wires 14-12, contactor 2-9 and rectifier 5 and wire iii-3 to main '7.

Upon closing contactor Zil, full wave rectified current in this circuit will go from 3 to 2 in the transistor and will render the transistor non-conducting.

assuming equal output potentials for the rectifiers S and o,the resistance of resistor R2 will be made less than that of R1. g

To iron out both half wave pulses in the rectified currents in these two circuits, so that the current will not go to zero value between the bait waves, inductors Z2 and 23 are provided in the respective circuits.

The inductors will delay the decay of each impulse so that it will be overlapped by the beginning of the next succeeding impulse and convert the unidirectional half waves into continuous unidirectional current.

At 24- is a main transformer, with its primary connected across the mains 7 and 8; and having a secondary 25 with a mid-point 26, from which a wire 27 goes to the said wire 14 and thence to the emitter 2 of the transistor.

A load 28 to be energized is connected in a line ii -29 between the collector 4 and the mid-point 3d, of the primary 31, of a feedback transformer 32.

The ends of the primary 31 are connected, respectively, by wires 33 34 through rectifier units35-36, to the ends of the main transformer secondary 25. V

The secondary 37 of the transformer 32 has a mid-point 38 connected by a wire 39 to the wire and thus to the base 3.

The ends of the secondary 37 are connected, respectively, through rectifier units 4t and d1 to a connection point 42, and thence through a resistor R3 to the wire 27.

At the main transformer 24 the halves of the secondary 25 are designated as t? and 454. a

At" the feedback transformer 32, the halves of the primary 31 are designated as 45 and id; and the halves of the secondary 37 are designated as 47 and In the operation of the system of FTGURE 1, all current to energize the load it; must go through the transistor; and normally, contactor Ill and contactor 2b are open; and the transistor it is nonconducting and no current flows to the load.

Upon closing contactor it, current flows over the above described circuit from line 8, by way of wire 1 ,7, rectifier 6, wire 1%, inductor 22, contactor 11, wire 12 and wire 14 into the emitter 2 and from the base 3 by way of wire 15, wire 16, resistor R1, rectifier 6 and wire 9 to line and this renders the transistor 1 conducting. 7

Current hen fiows from the mid-point 2d of the main transformer 24; by wires 27-and 14- to t e emitter 2 and out at the collector 4, and by wire 23 through the load 28, to the mid-point 3d of the primary 31 of the feedback transformer 32; and thence back to the main transformer secondary V in fuller explanation of this load energizing circuit, a half wave of alternating current in the main secondary 25 flowing, say, toward the right, can flow only in the half 43 of the secondary and into wire 27, since flow in the other half is blocked bythe rectifier 35.

The load current at the mid-point of the feedback transformer primary 31 will flow down through the primary half 45 and by wire 3 and rectifier 36 to the left end of the main secondary half l3.

Similarly, as will now be understood, the next half wave in the secondary 25, flowing toward the left, will flow in only the half 44 of the secondary to the wire 27; and at the feedback transformer, the flow from the mid-point 39 will be through rectifier and back to the half '44.

Both half waves from the main transformer 24 thus flow downwardly as unidirectional current through the transistor it and the load 28, upon rendering the transistor conducting by closing contactor 1131.

As explained in the premises it is a part of the invention that the transistor will be maintained conducting after opening the contactor ill, that is upon only momentary closing thereof. 'This is provided for as follows.

At the feedback transformer 32, the primary'halves 45 and 46 are, as described, alternately energized in correspondence with'the half waves of the alternating current supply. They accordingly induce potential in the secondary 3'7 alternately in opposite directions.

Potential in the up direction, corresponding to one half wave of the supply, can produce current in only the upper secondary half 48, current flow in the half 47 7 being blocked by the rectifier 411, and the current thus produced flows up in the half 43, through rectifier 4b, to point 42, through resistor R3 to Wire 27 and thence by wire 14, in at the emitter Z of the transistor, out at its base 3, and by Wire 15 to wire 39 and thence back to the midpoint 3d of the secondary.

Similarly as will now be understood, potential in the down direction in secondary 37 can produce current in only the lower'secondary half 57, which will flow downwardly, through the rectifier 41 and resistor R3 to wires 27 current supply there is an impulse of unidirectional current going into the transistor at its emitter 2 and out at its base 3, which will maintain it conducting. Such current, in the absence of countervailing provisions would go to zero value between each impulse and the next one, and to prevent this, an inductor 49 is placed in the line of the wire 27, through which the impulses must flow successively. The action of such an inductor to convert the half wave unidirectional impulses into continuous unidirectional current has been explained in connection with the inductors 22 and 23. V

The transistor will thus be maintained conducting, independently of the circuit controlled by the contactor 11 and this contactor 'rnaythen be restored. 7

It will thus be seen that only an impulse of current through the contactor 11 is needed, to set the system in condition to supply unidirectional full current to the load 28 from the alternating current supply mains 7-8.

To cut off the current to the load 28, or reduce it to zero value as of the premises the other contactor 20 which is normally in restored or open condition is operated to close its contacts.

It will first be assumed that the contactor 11 has been restored, or its contacts opened; and that the transistor is being maintained conducting by the load feedback transformer 32 as described; this being the condition for the mode of operation here described.

"Contacts 20 being operated, current flows from the supply line 7 over the above described path, wire 18, rectifier 5, resistor R2, wire 16, wire 15, to the base 3, and out at emitter 2, and by wire 14, wire 12 and wire 19, through contactor 2i), and by wire 21, rectifier and wire 21A to supply line 8.

This current in the transistor, as is well known, is in the direction to render the transistor non-conducting, and it may be made strong enough to do so and to overcome the conductivity maintaining current from the feedback transformer 32 above described, by giving a suitably loW value to the resistor R2; and the fiow of load current through the transistor stops.

Momentary closure of contactor 20, that is, a pulse only of current therethrough, is sufiicient to produce this result and contactor may now be restored, and the conditions of the system then obtaining will remain unchanged.

As a second mode of operation, the contactor 11 may be operated to set the system to supply full current to the load 28 as described, and thereafter be maintained operated. Then to reduce the load current to zero, the contactor 20 may be operated or its contacts closed.

As will be apparent from the foregoing description, current through the contactor 11 is in the direction to render the transistor conducting, and current through the contactor 20 is in the opposite direction, to render it nonconducting.

When contactor 20 is closed and contactor 11 is being maintained closed, the opposing current through contactor .20 is made great enough to overpower both the current through contactor 11 and that from the feedback transformer 32 and render the transistor non-conducting, and stop the load current.

This can be effected by providing the resistor R2 in the path of the contactor 20, with sufiiciently low resistance.

Upon again opening or restoring contactor 20, contactor 11 will still be maintained operated and the current therethrough will render the transistor conducting and the load current will again flow through it.

Thus by this second mode of operation, the load current can be reduced to zero by operating the contactor 20, and raised again to its full value by restoring the contactor 20, and so on alternately.

The difference in this mode of operation is that when the contactor 20 is closed, it must be maintained closed as long as zero load current is wanted, and not be closed only momentarily.

A modification of the above described system is illustrated in FIGURE 2, and explained by the saturation curve of FIGURE 3.

In FIGURE 2, some of the parts are the same as in FIGURE 1 and have been given the same reference nurnerals to identify them.

At is a closed magnetic circuit which functions in some respects like a transformer core, having a secondary winding 51 and two primary windings 52 and 53 thereon.

The secondary winding 51 has one end connected by wire 12 to a point 13 and thence by wire 14 to the emitter 2 of the transistor 1; and has the other end connected by Wires 16 and 15 to the transistor base 3.

The primary winding 52 is in acircuit connected to the alternating current supply mains 7-8. The circuit provides a current path through a wire 54 from the main 8, through a quadrilateral rectifier 55 to a wire 56 thence 6 through the winding 52 and through a normally open or restored contactor 57, when closed, and through an inductor 58 through the rectifier 55, and by wire 59 to the main 7.

The primary Winding 53 is in a similar circuit, providing a current path through a wire 60 from the main 7, through a rectifier 61 to a wire 62 thence through an inductor 63 and through a normally open or restored contactor 6 when closed, and through the winding 53 to a wire 65 and through the rectifier 61 and by a wire 66 to the main 8.

The primary windings 52 and 53 are wound and connected so that their magnetomotive forces will be in opposite directions, and preferably equal.

The material of the magnetic core 50 is chosen to have a hysteresis loop with characteristics such as illustrated in FIGURE 3. Magnetomotive force in the positive, or negative, direction (right or left) from the point 67, will efifect a rapid rise, or fall, of the flux to saturation, as at 63 or 69. The saturation value of flux remains substantially level when the magnetomotive force is removed, giving a high value of residual flux as at the points 79 or 71.

In a first mode of operation of the system of FIGURE 2 the contactors 57 and 64 are both normally open.

To cause the system to supply full current to the load by wire 29, the contactor 57 is operated or closed. Current from the lines 8 and 7, rectified at the rectifier 55 gives 'current to the primary winding 52 over the above described circuit.

Assuming that the core 56 has been saturated by previous operation and that its residual saturation flux is at the point 71 of FIGURE 3, the ampere turns of the primary winding 52, will go, from point 67 to point 72 in FIGURE 3, and will reverse the flux in the core 50 and raise it to saturation at the point 73.

This change of flux in the core 50 will generate an impulse of current in the secondary winding 51; the windings 52 and 51 being poled so that the current impulse flows upwardly in the secondary 51 and out on lines 12 and 14 to the emitter 2 of the transistor and out there from at the base 3 and back by line 16 to the secondary 51.

The transistor 1 is thereby rendered conducting, and as described for FIGURE 1 full load current flows in lines 27 and 14 through the transistor and by wire 29 to the load. The contactor 57 is then opened and the conductivity of the transistor is sustained by flow of current as in FIGURE 1, from the feedback transformer 32 along Wires 27 and 14, in at the emitter 2 and out at the base 3, and back to the feedback transformer 32 by wire 39.

Upon opening of contactor 57, tie-energizing primary 52, the flux, FIGURE 3, goes back from point 73 to point 70 and remains there as residual flux.

The saturation line 68 being almost level, very little change of flux and negligible impulse in Winding 51 occurs.

To reduce the load current to zero, contactor 64 is operated closing its contacts and by the above described circuit, energizes primary winding 53, the ampere turns thereof going, say, from point 67 to point 74 FIGURE 3.

Winding 53 being poled reversely with respect to winding 52, this gives a quick change of flux, in FIGURE 3, from point 70 to point 75; and this quick change of flux generates an impulse of current in winding 51 in the down direction, and it flows therefrom in Wires 16-15 to base 3 and out at emitter 2 and by wires 14-12 to secondary 51.

This as will be understood renders the transistor nonconducting, and load current ceases to flow therethrough.

Contactor 64 may now be opened or restored, and the flux will go to point 71 as residual flux; but again, due to the almost level characteristic of the saturation line 69, little change of flux value and a negligible impulse in winding 51 occurs.

Thus in FIGURE 1 by a first mode of operation, the contactor 11 is closed to give full load current, and is then opened; and the contactor 2a is closed to give zero load current, and then opened; and by a second mode of operation contactor 11 is closed to give full load current and is held closed, and coutactor 269 is closed and opened successively to give zero and full load current successively.

The first mode of operation of FIGURE 1 has also been described above modified by the arrangement in FIGURE 2, with primary windings 52 and 53 of opposite magnetomotive force, and preferably equal.

It now the second mode of operation as well as the first mode were wanted with FIGURE 1 modified by FIG- URE 2, then a primary winding 53 would be provided of greater magnetomotive force than that of the winding 52, say twice as much. T hen for the first mode of operation, when winding 53 is energized alone by contactor 64, to give zero load current, the contactor 57 being open, the first said mode of operation would be performed with the difierence that in FIGURE 3, the point '74 would betwice as far'from the point or as it is in FEGURE 3, and the point "75 would be twice as far along on the saturation curve 69 as the point '75; but the change in the flux producing an impulse in winding 51 would be that from point 7t) to point "f because the flux would change very little beyond the point '75; and the impulse would be little different from that above described for the first mode of operation with equal windings 53 and 52.

For the second mode of operation, to give zero cur rent to the load by closing contactor or with contactor 57 held closed, the flux at the time would he say, at the point 73 for the magnetomotive force of the winding 52 alone, at point 72.

Upon closing contactor 64 giving the double magnetomotive force of winding 53 the latter would predominate over that of winding 52 and shift the resultant magnetomotive force from point '72 to point 74- and the fiux at the point '73 would go to point '75 and produce the same impulse as in the first mode of operation, and upon successively closing and opening the contactor 64, with contactor 5'7 closed, the fiux'would go back and forth between point '25 and 73 and give successively opposite impulses in'the secondary S1 and cause the load current to change from zero to full value successively.

7 It is convenient for purposes of the claims, to refer to the circuit in the transistor from the terminal end of the emitter 2 to the terminal side of the base 3 as the transistor activating circuit or path, to render the transister conducting or non-conducting according to the direction of current in said path.

In FIGURE 4 is shown a system of the general type of FIGURE 2 but simplified and having a third mode of operation. it is arranged as an across-the-line class of diagrams.

Some of the parts have the same reference characters as in FIGURE 2 to identify them without further description.

The saturable transformer 5% has its secondary 51 always connected as in FIGURE 2 to the emitter 2 of the transistor 1 by wires 12 and l d and to the base 3 by wires 16 and 15. The conductivity maintaining cir-' cuit is shown at wire 27, wire 14, wire and wire 39.

A primary '76 is always connected across alternating current mains '7 and 3 through a rectifier 7'7 and is therefore always energized with half waves of the same polarity say positive.

A primary '78 is connected across the mains 7-8 through a rectifier '79 and through a contactor 8t) which 7 is normally closed; so that normally the primary 78 is energized with alternating current half waves which are synchronous with the half waves in the primary 7d.

The primaries '76 and '78 are connected so that their magnetomotive forces are in opposition; and the magnetomotive force of the primary '73 is greater than that of the primary '76, say twice as great.

Normally, when potential is first supplied'to the lines 7-8, both primaries 76 and '78 are simultaneously ener- Any impulse generated in the secondary 51 by the.

primary 73 as the flux rises, goes downwardly in the secondary 51 and in at the base 3 and out at the emitter 2; the secondary 51 beingconnected so that this will be the case; and the transistor will be non-conducting and no load current will flow through it. V

In operation, to give current to the load 28, the contactor it is opened. This dc-energizes primary 7%, leaving the primary '76 energized alone. i

The flux inFIGURE 3 then goes from'the saturation line 69, through the point it and abruptly to the saturation line 63, under the magnetornotive force of primary 76 as represented say by the point 72.

This rapid change of fruit generates current in the secondary 51 in the reverse direction, upwardly, the current going in atthe emitter 2 and out at the base 3 and rendering the transistor conducting; and load current ponderating value, which as described renders the tran sistor non-conducting.

in this form, FiGURE 4, as another mode of operation, with load current flowing, and maintaining the transistor conducting, the load current can be cut oil or reduced to zero by opening the feedback maintaining circuit, for example, by momentarily opening a contactor till in the line of the wire 39.

I claim as my invention:

ii. In an electric'control system, a transistor having a main and an activating circuit, an electric load connected to a source of current; the main circuit of said transistor connected between the load and source to all w current flow therethrough when conductive and to prohibit said current flow when non-conductive; a first circuit means connected to a source of current and comprising a normally open first contactor and arranged to cause current to flow in the activating circuit of said transistor in the direction to render said transistor conducting and cause current to flow in the load, upon closing the first contactor; sustaining circuit means arranged to energize the activating circuit in said transistor by current derived from current flowing in the load, and main taining said transistor conducting; a second circuit means connected to a current source and comprising 21 normally open second contactor and arranged to cause current to flow in the activating circuit of said transistor in the direction opposite to the aforesaid direction to render said transistor non-conducting and interrupt how of current in the load, upon closing the second contactor.

2. An electric system as described in claim 1 and in which, the sustaining circuit means maintains said transistor conducting independently of the first'circuit means,

main and an activating circuit, a main source of current connected to a load; the main circuit of said transistor connected between the load and the source to allow current fiow therethrough when conductive and to prohibit said current flow when non-conductive; a sustaining circuit arranged to derive current from the load current and connected to the activating circuit of said transistor to maintain it conducting when once it has been rendered conducting; a first circuit means comprising a first contactor connected to a current source and supplying current to the activating circuit of the transistor, to render it conducting, upon momentary closure of the first contactor; a second circuit means comprising a second contactor connected to a current source and supplying current to the activating circuit of the transistor in the direction to render the transistor non-conducting, upon momentary closure of the second contactor.

5. A control system as described in claim 4 and in which the main current source is a main transformer having a primary and a secondary winding; said primary winding connected to a pair of alternating current mains, and one side of the load is connected through the main circuit of said transistor to a mid-point of the main transformer secondary winding; and the other side of the load is connected to the mid-point of a primary winding of a feedback transformer, the opposite ends of said primary winding are connected to the opposite ends of the main transformer secondary winding; and the sustaining circuit is connected to a secondary winding of the feedback transformer and is supplied with current therefrom.

6. A control system as described in claim 4 and in which the first circuit means and second circuit means each comprises conductors connected at one end to the activating circuit of said transistor, and at their other ends connected to the corresponding current source, each of which provides unidirectional current operatively impressed on the conductors for transmission to the activating circuit respectively in opposite directions.

7. In an electric control system, a source of current; a load connected to receive current supplied by the source;

a transistor having a main current path therethrough in series with load, and said path being normally non-conducting; and the transistor having an activating current path therein which when energized renders the main path of the transistor conducting; a first contactor in a circuit connected to a current source and to the activating path and energizing it to render the transistor conducting when the contactor is closed, to thereby cause current to flow in the load; a maintaining circuit arranged to derive current from the load current and connected to the activating path of the transistor to maintain it activated; a second contactor in a second circuit connected to a current source and to the activating path and neutralizing activation of the transistor to render the transistor nonconducting when the second contactor is closed, and thereby interrupting the current in the load and in the maintaining circuit.

8. A control system as described in claim 7 and in which the activating path is maintained energized by the maintaining circuit after only momentary closure of the first contactor.

9. A control system as described in claim 7 and in which the activation of the transistor is neutralized upon closure of the second contactor whether or not the first contactor continues to be closed.

References Cited by the Examiner UNITED STATES PATENTS 2,774,888 12/56 Trousdale 307-885 2,808,990 10/57 Van Allen 307-885 2,831,126 4/58 Linvill et a1 307-885 2,877,360 3/59 Moore et al. 307-885 2,931,921 4/60 Smeltzer et al. 307-885 LLOYD MCCOLLUM, Primary Examiner. 

7. IN AN ELECTRIC CONTROL SYSTEM, A SOURCE OF CURRENT; A LOAD CONNECTED TO RECEIVE CURRENT PATH THERETHROUGH IN A TRANSISTOR HAVING A MAIN CURRENT PATH THERETHROUGH IN SERIES WITH LOAD, AND SAID PATH BEING NORMALLY NON-CONDUCTING; AND THE TRANSISTOR HAVING AN ACTIVATING CURRENT PATH THEREIN WHICH WHEN ENERGIZED RENDERS THE MAIN PATH OF THE TRANSISTOR CONDUCTING; A FIRST CONTACTOR IN A CIRCUIT CONNECTED TO A CURRENT SOURCE AND TO THE ACTIVATING PATH AND ENERGIZING IT TO RENDER THE TRANSISTOR CONDUCTING WHEN THE CONTACTOR IS CLOSED, TO THEREBY CAUSE CURRENT TO FLOW IN THE LOAD; A MAINTAINING CIRCUIT ARRANGED TO DERIVE CURRENT FROM THE LOAD CURRENT AND CONNECTED TO THE ACTIVATING PATH OF THE TRANSISTOR TO MAINTAIN IT ACTIVATED; A SECOND CONTACTOR IN A SECOND CIRCUIT CONNECTED TO A CURRENT SOURCE AND TO THE ACTIVATING PATH AND NEUTRALIZING ACTIVATION OF THE TRANSISTOR TO RENDER THE TRANSISTOR NONCONDUCTING WHEN THE SECOND CONTACTOR IS CLOSED, AND THEREBY INTERRUPTING THE CURRENT IN THE LOAD AND IN THE MAINTAINING CIRCUIT. 